1. Field of the Invention
The invention relates to the field of control devices, especially a polarization state control device for optical communication systems and other uses, wherein two or more parameter controls are cascaded, each variably contributing to the value of an output, and each control having only a finite span of control range. The invention provides a technique for coordinating the extent of displacement of the output value that is contributed by the respective parameter controls. The technique is useful for an endless polarization state controller (the polarization state being a set of periodic attributes), and is also applied in this disclosure to a polarization scrambler and a polarization synthesizer.
2. Prior Art
It may be desirable in various contexts to have two or more independently controlled parameters contribute to determining the value of an output. In a positioning control, for example, two controllers each having a span of zero to “X” might be cascaded, i.e., one carried on the other, to achieve a summed span of zero to 2X. At the limit of the span (near 2X), it is necessary to have each cascaded parameter set near its full 1X span. At output values that are less than the full span, more or less of the output can be contributed by one or the other of the cascaded controls, provided that the sum is equal to the desired output. Thus, for example, an output value of 1X could be achieved by any arbitrary combination adding up to the desired 1.0 value (e.g., 1.0+0.0 or 0.5+0.5 or 0.62+0.38, etc.).
Polarization controllers are akin to positioning controls because the polarization state of a propagating electromagnetic wave such as a light beam or light wave is due in part to the presence and/or relative amplitude of mutually orthogonal components of the electric field, and also is due in part to the relative phase positions of these components. Inserting a delay or retardation of one component relative to the other, thereby effectively repositioning the components relative to one another, amounts to a change in the polarization state of the light wave.
The periodicity of the light wave is another factor to consider. Assuming that one of the mutually orthogonal components at a particular wavelength is delayed relative to the other by an integer multiple of 2π radians or 360°, the same polarization state is achieved as if there was a no delay or a delay of some other integer multiple.
It may be desirable to provide a polarization control with an output span that is greater than 2π radians at a given wavelength, for example simply to achieve a zero to 2π span at some longer wavelength or to mimic the endless rotation of a waveplate. In a differential sort of control, it may be desirable to provide a controller that is always capable of adding to the retardation of one of the components, regardless of the previous value of the retardation at which the control was set. That capability effectively requires a control span from zero to infinity.
Relative retardation of mutually orthogonal components can be controlled using controllable birefringence such as electrically controlled liquid crystals. By definition, birefringence involves a difference in the retardation experienced by mutually orthogonal components of a light signal. Controlling the birefringence determines the difference in the respective retardations and thus the relative phase delay. With a suitable arrangement of at least two plates oriented at 45° relative to one another (so as to ensure controllable retardation of each component of an arbitrary input), the polarization state of a light signal can be adjusted.
It may be possible to provide a birefringent element, such as a liquid crystal, that is thick enough to provide the desired span of relative retardation control (e.g., zero to 360° or some other span). For practical reasons, it may be preferable to have plural cascaded birefringent elements that contribute additively to the retardation. As an example, the response time of two thin elements that additively achieve a given retardation is likely to be less than the response time of a single thick element that is capable of the same retardation.
Polarization controllers using tunable birefringent waveplates are known. Typical configurations use three cells (see, e.g., U.S. Pat. No. 4,979,235) or four cells (U.S. Pat. No. 5,005,952). These configurations may suffer from problems that arise because limited tuning range devices, namely individual cascaded controls, are being used to control an unlimited periodically repeating parameter, namely polarization evolution. Assuming that a control value is incrementally increased, for example, continued operations may call for an unlimited or infinite span of control, even though the phase delay repeats at increments of 2π radians. This conflict can be termed the “endless control” problem. The required parameter variations that may be required continue endlessly, but the individual cascaded elements have only a finite span of which they are capable of differentially retarding components of the light signal. As a consequence, the present control values applied to the individual elements may need to be brought back from their limits, resetting operations at a new starting point, when a limit is reached for one or more control spans. The limit has made further increase in that control value impossible.
If one differentially adds an increment of retardation in repetitive control steps to one or another of the controlled cascaded elements, one will eventually reach the end of the span of that controlled element. It is then necessary to interrupt the control procedure (i.e., to stop simply adding increments to the now-maxed controlled element) and to achieve the desired output in a different way.
There are methods that can deal with this problem, but they often involve complicated structures and synchronized mechanical and electrical controls. Complication and expense are normally to be avoided in a practical application. One method is to choose a sort of control that inherently has an unlimited tuning range, such as one comprising rotating waveplates. The rotational position of a waveplate can provide a form of unlimited free space optical orientation that is unlimited, unlike a fixed-span adjustable device such as a birefringent element with a finite span of controllable birefringence. Nothing prevents indefinitely continued rotation of a rotatable waveplate, in one direction or the other. A mechanical polarization rotator is available from OZ Optics, Ltd., Ottawa, ON. An electrical polarization rotator with specially designed electrodes is described by T. Chiba, Y. Ohtera, and S. Kawakami, Journal of Lightwave Technology, Vol. 17, No. 5, p885, 1999. These solutions are less than optimal. The mechanical rotation technique is susceptible to fatigue and it is bulky. The electrical rotation technique relies on precise alignment and high control voltages.
An alternative method for dealing with the endless problem is to use “unwinding” of one or more cascaded controls of a periodic parameter. Unwinding has the object of bringing the control point back, from a point that is at or uncomfortably close to the end of its control range. In order to obtain the same value at the output, the control point can be returned to a point that differs by a retardation of precisely 2π, typically by varying the level of other contributing controls (e.g., controls that are cascaded with the one being unwound). This must be done in a coordinated manner in order to maintain the desired output level of the contributed or cascaded control parameters while the one is being unwound. Because the output polarization state is insensitive to a 2π change in retardation, the contributing controls will return to their respective previous values after the “unwinding” process.
An example of this unwinding or resetting process as described can be found in the publication of S. H. Rumbaugh, M. D. Jones, and L. W. Casperson, Journal of Lightwave Technology, Vol. 8, No. 3, p459, 1990. In that paper, an endless polarization control scheme is described using three Liquid Crystal cells. The authors discuss an endless transformation, namely from an arbitrary state of polarization to a fixed linear state of polarization.
The general principle of unwinding was also reported in a paper of N. G. Walker and G. R. Walker, Journal of Lightwave Technology, Vol. 8, No. 3, p438, 1990. The configuration and unwinding mechanism described in this paper require birefringent elements having a retardation tuning range of 4π, or two complete 360° phase delays, in order to achieve unwinding in any situation. In some situations, the unwinding process is complex. It would be advantageous if unwinding could be accomplished without requiring such a large span of controllable retardation, thereby employing relatively thinner, faster and possibly less costly elements.
What is needed is a way to manage the span of cascaded control parameters, so as to deal efficiently with the problem of unwinding a control that has reached an undesired point in its control range.